New Family of Dinuclear\nNi–Ln Complexes from\nTernary Coordination by β‑Diketonates around the Lanthanoids:\nSynthesis, Structures, and Magnetic Behavior with Theoretical Supports

A single-pot\nsynthetic endeavor was utilized with a binucleating\nphenolate-centered unsymmetrical ligand to obtain two new series of\nNi–Ln complexes. The method also facilitated the capping of\ntwo types of β-diketonate anions around larger lanthanide ions\nwith vacant coordination sites. Using dibenzoylmethane (dbmH) and\nbenzylacetonate (bzaH) as ancillaries, two distinct groups of complexes,\n[NiLn(HL)<sub>2</sub>(dbm)<sub>2</sub>]NO<sub>3</sub> (where Ln =\nTb<sup>3+</sup> (<b>1a</b>), Dy<sup>3+</sup> (<b>1b</b>), and Ho<sup>3+</sup> (<b>1c</b>)) and [NiLn(HL)<sub>2</sub>(bza)<sub>2</sub>]NO<sub>3</sub>·<i>x</i>MeOH (where\nLn = Gd<sup>3+</sup> (<b>2a</b>), <i>x</i> = 3; Tb<sup>3+</sup> (<b>2b</b>), <i>x</i> = 0; Dy<sup>3+</sup> (<b>2c</b>), <i>x</i> = 0; and Er<sup>3+</sup> (<b>2d</b>), <i>x</i> = 0) (H<sub>2</sub>L = 2-methoxy-6-[(<i>E</i>)-2′-hydroxymethyl-phenyliminomethyl]-phenol), were\nobtained in good yields. For complexes <b>1a</b>–<b>2d</b>, two HL<sup>–</sup> units grab Ni<sup>II</sup> ions\nfirst in an elongated octahedral N<sub>2</sub>O<sub>4</sub> geometry\nwith <i>apical</i> coordination of two alcohol OH groups.\nThe Ln<sup>III</sup> centers were next caged in distorted O<sub>8</sub> donor-based square antiprism geometry. Both dc and ac magnetic susceptibility\nmeasurements demonstrated (i) the involvement of ferromagnetic interaction\nbetween the adjacent nickel(II) (<i>S</i> = 1) and other\nlanthanoids except Ho<sup>III</sup> and (ii) a lack of slow magnetic\nrelaxation at 2 K in zero applied dc field. The computational study\nusing <i>ab initio</i> methods involving the CASSCF (Complete\nActive Space Self-Consistent Field), RASSI-SO (Restricted Active Space\nState Interaction Spin–Orbit), and SINGLE_ANISO programs, along\nwith the POLY_ANISO program, provides detailed insights into the magnetic\nproperties and relaxation mechanisms of single-ion systems, particularly\nin lanthanide complexes. Theoretical calculations backed the experimental\nresults of the lack of slow relaxation of magnetization for <b>1a</b>–<b>1c</b> and <b>2b</b>–<b>2d</b> at zero dc field and lack of prominent maxima for the out-of-phase\ncomponent of the magnetic susceptibility in the applied dc field.\nThe weak Ni–Ln exchange and the noncollinearity between the\nanisotropic axes of Ni<sup>II</sup> and Ln<sup>III</sup> ions led\nto the significant tunneling gap (Δ<sub>tun</sub>) and the QTM\nbetween the ground exchange coupled doublet states in all the complexes\nthat directed the magnetic relaxation even in the ground state.